The Treatment of Missing Values in Logistic Regression

The efficiencies of the estimators in the linear logistic regression model are examined using simulations under six missing value treatments. These treatments use either the maximum likelihood or the discriminant function approach in the estimation of the regression coefficients. Missing values are assumed to occur at random. The cases of multivariate normal and dichotomous independent variables are both considered. We found that in general, there is no uniformly best method. However, mean substitution and discriminant function estimation using existing pairs of values for correlations turn out to be favourable for the cases considered.     

Clutch size relative to tree cavity size in Northern Flickers

We analysed clutch size versus nest size in 153 broods of the Northern Flicker Colaptes auratus , a woodpecker using natural cavities in British Columbia, Canada. Larger volume cavities were less susceptible to predation and cavity size was positively associated with the age and body size of males and with the body condition of female parents. Although clutches varied between 4 and 11 eggs, and the floor area of cavities varied about 5-fold, we found no relationship between clutch size and floor area or cavity volume. To see if there were fitness consequences to clutch size relative to nest size, we examined hatching success and nestling mortality in flicker broods. Hatching success was not related to cavity size, but crowding slightly reduced nestling survival even when clutch size was controlled statistically. However, there was no effect of cavity size on the total number of nestlings fledged. Newly excavated flicker cavities were smaller than reused cavities suggesting a cost to excavation. This cost, coupled with the minimal fitness consequences of overcrowding, may explain why flickers do not adjust clutch size to cavity size.     

Biosynthesis and secretion of the rat core-specific lectin. Relationship of post-translational modification and assembly to attainment of carbohydrate binding activity

A soluble lectin, the core-specific lectin (CSL), is synthesized and secreted by rat hepatocytes and the rat hepatoma cell line, H-4-II-E. This lectin binds mannose and N-acetylglucosamine residues in the "core" region of Asn-linked oligosaccharides. Secretion of the CSL was found to occur over an extended period of time, greater than 4 h being required for secretion of 50% of the lectin (Brownell, M. D., Colley, K. J., and Baenziger, J. U. (1984) J. Biol. Chem. 259, 3925-3932). We have determined that following synthesis in the endoplasmic reticulum, the CSL is rapidly transported to the Golgi where it is retained for an extended period of time prior to secretion. The lectin undergoes two post-translational modifications within the Golgi: an increase from Mr 24,000 to 25,000 and a progressive decrease in pI with an accompanying increase in Mr to a final value of 26,000. The lectin is also assembled into high molecular weight complexes of 150-260 X 10(3) and acquires the ability to bind carbohydrate in the Golgi. In hepatoma cells, the 24,000-25,000 modification is completed 20 min after initiation of synthesis. Assembly of the CSL subunits into high molecular weight complexes, acquisition of carbohydrate binding activity, and the 25,000-26,000 modification occur between 20 and 80 min after initiation of synthesis. These events have slower kinetics in primary hepatocytes and this allowed us to determine that the sequence of these biosynthetic events is: the 24,000-25,000 modification, complex assembly, the 25,000-26,000 modification, and acquisition of carbohydrate binding activity. The 24,000-25,000 modification occurs prior to complex assembly. Complex assembly may occur prior to, or concomitant with, the 25,000-26,000 modification. Assembly into the oligomeric form and the 25,000-26,000 modification correlate with the attainment of carbohydrate binding activity. The kinetics of CSL modification and assembly cannot account for its retention within the Golgi. Interaction with Golgi components either through carbohydrate binding or another interaction, may act to selectively retain the lectin within the Golgi.     

Post-translational modifications of the core-specific lectin. Relationship to assembly, ligand binding, and secretion

The rat core-specific lectin (CSL) or mannan-binding protein is synthesized and secreted by rat hepatocytes and H-4-II-E hepatoma cells. Prior to secretion proline and lysine residues with collagen-like sequences undergo hydroxylation and subsequent glycosylation of hydroxylysine to produce glucosylgalactosylhydroxylysine. Hydroxylation and subsequent glycosylation are inhibited by alpha,alpha'-dipyridyl (Colley, K. J., and Baenziger, U. U. (1987) J. Biol. Chem. 262, 10290-10295). We have used alpha,alpha'-dipyridyl to investigate the role of hydroxylation and glycosylation on interchain disulfide bond formation, assembly of subunits into high molecular weight complexes, attainment of carbohydrate and lipid binding ability, and secretion. Formation of disulfide-bonded dimers and trimers in the endoplasmic reticulum, assembly into high molecular weight complexes in the Golgi, and attainment of carbohydrate binding activity occur in either the presence or absence of these post-translational modifications. The mature fully processed form of the CSL binds hydrophobic matrices and is secreted at a slow, but linear, rate. Inhibition of proline and lysine hydroxylation and hydroxylysine glycosylation prevents CSL secretion and attainment of binding activity for hydrophobic matrices. Secretion of the lectin, although slow, appears to be an active process and may be related to the capacity to interact with membranes and/or lipids. Other proteins known to contain collagen-like sequences such as acetylcholinesterase, pulmonary surfactant apoproteins, and C1q also interact with lipids and/or membranes. The collagen-like domains of these proteins may also play a role in promoting such interactions.     

Identification of the post-translational modifications of the core-specific lectin. The core-specific lectin contains hydroxyproline, hydroxylysine, and glucosylgalactosylhydroxylysine residues

The core-specific lectin (CSL) synthesized and secreted by rat hepatocytes and the rat hepatoma H-4-II-E shows affinity for mannose and N-acetylglucosamine residues in the "core" region of asparagine-linked oligosaccharides. The CSL undergoes two stages of post-translational modification which result in an increase in its Mr from 24,000 to 26,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. We have determined that the lectin undergoes hydroxylation of proline and lysine and that the hydroxylysine is glycosylated to form glucosylgalactosylhydroxylysine (GlcGalHyLys). CSL metabolically labeled with [3H]lysine and [3H]proline contains hydroxylated forms of proline and lysine. The mature form of the lectin can also be metabolically labeled with [3H]galactose. alpha,alpha'-Dipyridyl, an inhibitor of collagen prolyl and lysyl hydroxylases, prevents the metabolic incorporation of [3H]galactose and the post-translational increases in the Mr of the CSL, indicating that both events are dependent upon hydroxylation of proline and lysine. Virtually all of the hydroxylysine present in the CSL is recovered as glucosylgalactosylhydroxylysine after alkaline hydrolysis. The post-translational modifications of the CSL place it in a select family of secreted proteins which contain collagen-like sequences, including the pulmonary surfactant proteins, complement component C1q, and the 18 S asymmetric form of acetylcholinesterase.     

Abnormal subcellular distribution of β-glucuronidase in mice with a genetic alteration in enzyme structure

Liver -glucuronidase is structurally altered in inbred strain PAC so that a peptide subunit with a more basic isoelectric point, GUS-SN, is produced. This allele of -glucuronidase was transferred to strain C57BL/6J by 12 backcross matings to form the congenic line B6 · PAC-Gus ⁿ. Liver -glucuronidase activity was halved in males of the congenic strain compared to normal males. The lowered activity was specifically accounted for by a decrease in the lysosomal component. There was no alteration in the concentration of microsomal activity. This alteration in the subcellular distribution of -glucuronidase in Gus ⁿ/Gus ⁿ mice was confirmed by two independent gel electrophoretic systems which separate microsomal and lysosomal components. -Glucuronidase activity was likewise approximately halved in mutant spleen, lung, and brain, organs which contain exclusively or predominantly lysosomal -glucuronidase. The loss of liver lysosomal -glucuronidase activity was shown by immunotitration to be due to a decrease in the number of -glucuronidase molecules in lysosomes of the congenic strain. The Gus ⁿ structural alteration likely causes the lowered lysosomal -glucuronidase activity since the two traits remain in congenic animals. Heterozygous Gus ⁿ/Gus ^b animals had intermediate levels of liver -glucuronidase. Also, the effect was specific, in that three other lysosomal enzymes were not reproducibly lower in Gus ⁿ/Gus ⁿ mice. Gus ⁿ is, therefore, an unusual example of a mutation which causes a change in the subcellular distribution of a two-site enzyme.This work was supported by National Institutes of Health Grants GM-33559 and GM-33160 and National Science Foundation Grant PCM-8215808.     

Occurrence of bacterivory in Cryptomonas,a common freshwater phytoplankter

Summary Bacterivory was detected by incorporation of 0.57 m diameter, fluorescent polystyrene beads and fluorescently labeled bacteria (FLB) in two cultured species of Cryptomonas (C. ovata and C. erosa), and a population of Cryptomonas sp in a humic, mesotrophic lake. Rates of ingestion and clearance were very low, and similar for the cultures and the in situ population. The in situ population incorporated 0.7–1.7 bacteria cell^-1 h^-1, thereby ingesting 0.3%–2.0% of the total bacterial numbers present in the water per day, and receiving less than 2% of its carbon content per day through bacterivory. Thus, bacterivory by Cryptomonas was quantitatively important neither as a sink for bacterial biomass, nor as a carbon source for the algal cells. Possibly, it served in the uptake of essential nutrients.